Setting: Scotland, 1700
Near Lammermoor Castle, guards led by Normanno prepare to search the woods for a mysterious trespasser. Enrico, Lucia’s brother, joins Normanno, enraged regarding Lucia’s refusal to marry Arturo. When Raimondo suggests that the death of her beloved mother is keeping her from thoughts of love, Normanno says that Lucia has been seen with another man. After his identity is revealed as Edgardo, Enrico flies into a fit of rage and swears vengeance against him and Lucia.
Meanwhile, by a fountain near her mother’s tomb, Lucia waits with her friend Alisa for a visit from Edgardo. Despite Alisa’s pleas for caution, Lucia remains strong in her love for Edgardo. Edgardo arrives to tell her he must leave for France on political business but vows to make amends with Enrico to make her his wife. Instead she makes him promise to keep their love a secret. He reluctantly agrees, and the two exchange rings and love vows before bidding each other farewell.
At the castle, Enrico and Normanno plan the deception that will force Lucia to marry Arturo. They have intercepted all the letters sent by Edgardo to Lucia and crafted a letter to her that proves Edgardo loves another. As Normanno leaves, Lucia enters and Enrico shows her the forgery. Heartbroken and destroyed, Lucia longs only for death, but she is encouraged by both Enrico and Raimondo to think of the family and marry Arturo. Raimondo assures Lucia that heavenly rewards await her for such earthly sacrifice.
In the castle’s great hall, guests celebrate the impending union. Preparing Arturo for Lucia’s behavior, Enrico claims she is still mourning the death of her mother. Lucia is led in, clearly distraught. She has just finished signing the marriage contract when Edgardo rushes in. Bloodshed is narrowly avoided, but when Edgardo sees Lucia’s signature on the marriage contract, he hastily removes his ring from her hand and leaves, cursing her. Lucia collapses in agony.
As a storm rages, Edgardo sits alone in Wolf’s Crag Tower. Enrico finds him and challenges him to a duel at dawn. They are to meet at the tombs of the Ravenswoods.
Meanwhile, the wedding celebrations are halted when Raimondo enters to say that Lucia has stabbed and killed Arturo in the wedding chamber. Lucia stumbles in, obviously mad and unaware of what she has done, rambling about Edgardo and imagining that she is married to him. Enrico enters, ready to berate Lucia for her crime, but he is silenced by the sight of her condition. As he repents, she collapses.
As Edgardo waits by the tombs of his ancestors, he laments Lucia’s supposed betrayal and wishes for his own death. Guests leaving Lammermoor Castle tell him about Lucia’s actions and that she has collapsed, crying his name. He leaves to rush to her side, only to be told by Raimondo that it is too late. Realizing that her love for him never waned, Edgardo vows to join Lucia in heaven. He removes his sword, stabs himself, and dies.